The cardiovascular and respiratory effects of medetomidine and thiopentone anaesthesia in dogs breathing at an altitude of 1486 m.

K E Joubert,R Lobetti

doi:10.4102/jsava.v73i3.569

Abstract

The purpose of this study was to evaluate the cardio-respiratory effects of the combination of medetomidine and thiopentone followed by reversal with atipamezole as a combination for anaesthesia in 10 healthy German Shepherd dogs breathing spontaneously in a room at an altitude of 1486 m above sea level with an ambient air pressure of 651 mmHg. After the placement of intravenous and intra-arterial catheters, baseline samples were collected. Medetomidine (0.010 mg/kg) was administered intravenously and blood pressure and heart rate were recorded every minute for 5 minutes. Thiopentone was then slowly administered until intubation conditions were ideal. An endotracheal tube was placed and the dogs breathed room air spontaneously. Blood pressure, pulse oximetry, respiratory and heart rate, capnography, blood gas analysis and arterial lactate were performed or recorded every 10 minutes for the duration of the trial. Thiopentone was administered to maintain anaesthesia. After 60 minutes, atipamezole (0.025 mg/kg) was given intramuscularly. Data were recorded for the next 30 minutes. A dose of 8.7 mg/kg of thiopentone was required to anaesthetise the dogs after the administration of 0.010 mg/kg of medetomidine. Heart rate decreased from 96.7 at baseline to 38.5 5 minutes after the administration of medetomidine (P < 0.05). Heart rate then increased with the administration of thiopentone to 103.2 (P < 0.05). Blood pressure increased from 169.4/86.2 mmHg to 253.2/143.0 mmHg 5 minutes after the administration of medetomidine (P < 0.05). Blood pressure then slowly returned towards normal. Heart rate and blood pressure returned to baseline values after the administration of atipamezole. Arterial oxygen tension decreased from baseline levels (84.1 mmHg) to 57.8 mmHg after the administration of medetomidine and thiopentone (P < 0.05). This was accompanied by arterial desaturation from 94.7 to 79.7% (P < 0.05). A decrease in respiratory rate from 71.8 bpm to 12.2 bpm was seen during the same period. Respiratory rates slowly increased over the next hour to 27.0 bpm and a further increases 51.4 bpm after the administration of atipamezole was seen (P < 0.05). This was maintained until the end of the observation period. Arterial oxygen tension slowly returned towards normal over the observation period. No significant changes in blood lactate were seen. No correlation was found between arterial saturation as determined by blood gas analysis and pulse oximetry. Recovery after the administration of atipamezole was rapid (5.9 minutes). In healthy dogs, anaesthesia can be maintained with a combination of medetomidine and thiopentone, significant anaesthetic sparing effects have been noted and recovery from anaesthesia is not unduly delayed. Hypoxaemia may be problematic. Appropriate monitoring should be done and oxygen supplementation and ventilatory support should be available. A poor correlation between SpO2 and SaO2 and ETCO2 and PaCO2 was found.

Highlights

Medetomidine has been used as a premedication for thiopentone anaesthesia[1,29]
In dogs sedated with medetomidine and breathing 50 % oxygen, it was demonstrated that an increase in alveolar-arterial oxygen difference occurred[1]
The blood pressure transducer was calibrated to a standard mercury column and the pressure transducer was adjusted to the height of the sternum before baseline samples were collected for heart and respiratory rates, blood pressure, blood gas analysis and arterial lactate

Summary

Introduction

Medetomidine has been used as a premedication for thiopentone anaesthesia[1,29]. It has been recommended that medetomidine should not be used in critical patients, as the lowered dose of thiopentone does not necessarily balance the negative effects of medetomidine on the cardiovascular and respiratory systems[1]. In dogs sedated with medetomidine and breathing 50 % oxygen, it was demonstrated that an increase in alveolar-arterial oxygen difference occurred[1]. Dogs breathing room air may have an aSection of Anaesthesiology, Department of Companion Animal Clinical Studies, Faculty of Veterinary Science, University of Pretoria, Private Bag X04, Onderstepoort, 0110 South Africa. BSection Small Animal Medicine, Department of Companion Animal Clinical Studies, Faculty of Veterinary. Medetomidine and thiopentone are used in private practice, either on their own or in combination, for routine procedures such as radiographs, gastroscopy, and minor surgical procedures. There are reports of hypoxia, cyanosis and acute cardiovascular collapse as well as speculation that this combination may be contra-indicated in the dog[1,8,29]. There are few reported studies of the use of medetomidine and thiopentone in combination

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Conclusion